That's too many hermits and too many emeralds for a 55. You better keep them well fed or they most certainly will eat the snails. 20 astraeas is a lot to look after. They do not have the righting mechanism and are an easy crab buffet if you aren't doing a head count at least twice a day. The hermits won't really eat them for their shell because it's not their preferred shape.... but with that many hermits I'm sure the risk is huge.
I don't keep hermits personally as a part of my CUC. But I do have 2 tanks that have a scarlet. Where they are not there to function as a part of my CUC, I keep them well fed. They are each living with a cerith, which is one of the preferred shell shapes, but have not touched them.I think their shells are too small. I just threw a conch shell in there and he moved on in.
In a month or two... or three your CUC popoluation is going to be cut in to 3/4 - 1/2, hermits included.